Uninterruptible power supply (UPS) systems with economy modes of operation typically create voltage disturbances on the output voltage when transitioning between modes of operation. For example, if a utility source line to ground fault condition occurs while in an economy mode, there will be a severe voltage sag until such time that the system transitions to a normal mode of operation. The temporary voltage disturbance would normally be interpreted by a static transfer switch as a source failure and thereby result in a transfer to an alternate source by the static transfer switch. In some arrangements, the transfer to the alternate source may be undesirable. For example, if the alternate source is a standby source shared by several static transfer switches, resources of the standby source may need to be kept in reserve for more critical applications. Therefore, it is desirable to improve operation of a static transfer switch during voltage disturbances, including those created by an upstream UPS system.
This section provides background information related to the present disclosure which is not necessarily prior art.